U.S. patent number 8,909,335 [Application Number 13/967,878] was granted by the patent office on 2014-12-09 for method and apparatus for applying a rectilinear biphasic power waveform to a load.
This patent grant is currently assigned to Zoll Medical Corporation. The grantee listed for this patent is Zoll Medical Corporation. Invention is credited to James G. Radzelovage.
United States Patent |
8,909,335 |
Radzelovage |
December 9, 2014 |
Method and apparatus for applying a rectilinear biphasic power
waveform to a load
Abstract
A system and method to deliver a therapeutic quantity of energy
to a patient. The system includes a capacitor having a rated energy
storage capacity substantially equal to the therapeutic quantity of
energy, a boost converter coupled with the capacitor and
constructed to release energy from the capacitor at a substantially
constant current for a time interval, and an H-bridge circuit
coupled with the boost converter and constructed to apply the
substantially constant current in a biphasic voltage waveform to
the patient. The method includes storing a quantity of energy
substantially equal to the therapeutic quantity of energy in a
capacitor, releasing the quantity of energy at a relatively
constant current during a time interval using a boost converter
coupled with the capacitor, and delivering a portion of the
quantity energy in a direction to the patient using an H-bridge
circuit coupled with the boost converter.
Inventors: |
Radzelovage; James G.
(Londonderry, NH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Zoll Medical Corporation |
Chelmsford |
MA |
US |
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Assignee: |
Zoll Medical Corporation
(Chelmsford, MA)
|
Family
ID: |
50100591 |
Appl.
No.: |
13/967,878 |
Filed: |
August 15, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140052204 A1 |
Feb 20, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61691137 |
Aug 20, 2012 |
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Current U.S.
Class: |
607/5 |
Current CPC
Class: |
A61N
1/3981 (20130101); A61N 1/362 (20130101); A61N
1/3912 (20130101) |
Current International
Class: |
A61N
1/00 (20060101) |
Field of
Search: |
;607/5 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report and Written Opinion of the
International Searching Authority, or the Declaration for
International Application No. PCT/US13/55110, mailed Jan. 17, 2014,
14 pages. cited by applicant.
|
Primary Examiner: Manuel; George
Attorney, Agent or Firm: Lando & Anastasi, LLP
Parent Case Text
RELATED APPLICATIONS
This application claims priority under 35 U.S.C. .sctn.119(e) to
U.S. Provisional Application Ser. No. 61/691,137, titled "METHOD
AND APPARATUS FOR APPLYING A RECTILINEAR BIPHASIC POWER WAVEFORM TO
A LOAD," filed on Aug. 20, 2012, which is hereby incorporated
herein by reference in its entirety.
Claims
What is claimed is:
1. A system to deliver a therapeutic quantity of energy to a
patient load, the system comprising: a capacitor having a rated
energy storage capacity substantially equal to the therapeutic
quantity of energy; a boost converter coupled with the capacitor
and constructed to release energy from the capacitor at a
substantially constant current for a time interval, the boost
converter comprising an inductor coupled with the capacitor, a
current sensing network, and a solid-state switch coupled between
the inductor and the current sensing network; and an H-bridge
circuit coupled with the boost converter and constructed to apply
the substantially constant current in a biphasic voltage waveform
to the patient load.
2. The system of claim 1, wherein the boost converter further
comprises a controller circuit coupled with the solid state switch
and the current sensing network and constructed to cycle the solid
state switch.
3. The system of claim 2, wherein the current sensing network is
constructed to receive a current profile and compare the current
profile with a received current from the solid state switch.
4. The system of claim 1, wherein the H-bridge circuit comprises a
plurality of switches, each of the plurality of switches including
a circuit constructed to control the switch and to receive a phase
profile having a first phase and a second phase.
5. The system of claim 4, wherein the H-bridge circuit further
comprises an inverter coupled with at least two of the plurality of
switches to invert the phase profile.
6. The system of claim 5, wherein at least two switches of the
plurality of switches are configured to be in an open state during
the first phase and in a closed state during the second phase.
7. The system of claim 1, wherein the boost converter circuit is
further constructed to compensate for voltage droop on the
capacitor and variation in the patient load over the time
interval.
8. The system of claim 1, wherein the therapeutic quantity of
energy and the time interval are selected to defibrillate a patient
whose heart is in fibrillation.
9. The system of claim 1, wherein the therapeutic quantity of
energy and the time interval are selected to pace a patient whose
heart requires pacing impulses.
10. A method of delivering a therapeutic quantity of energy to a
patient load, the method comprising: storing a quantity of energy
substantially equal to the therapeutic quantity of energy in a
capacitor; releasing the stored quantity of energy at a relatively
constant current during a time interval using a boost converter
coupled with the capacitor by transferring energy to an inductor
coupled with the capacitor and sensing an amount of current through
a solid state switch coupled between the inductor and a current
sensing network; and delivering a first portion of the stored
quantity of energy in a first direction to the patient load using
an H-bridge circuit coupled with the boost converter.
11. The method of claim 10, wherein the method further comprises
delivering a second portion of the stored quantity of energy in a
second direction to the patient load using the H-bridge
circuit.
12. The method of claim 11, wherein the H-bridge circuit comprises
a plurality of switches and wherein delivering a first portion of
the stored quantity energy in a first direction and a second
portion of the stored quantity of energy in a second direction
includes controlling the plurality of switches.
13. The method of claim 12, wherein controlling the plurality of
switches includes receiving a phase profile having a first phase
and a second phase.
14. The method of claim 13, wherein controlling the plurality of
switches further includes changing a state of at least 4 switches
of the plurality of switches in response to receiving a change in
the phase profile from the first phase to the second phase.
15. The method of claim 10, wherein releasing the stored quantity
of energy further includes cycling the solid state switch using a
controller circuit coupled with the solid state switch and the
current sensing network.
16. The method of claim 15, wherein releasing the stored quantity
of energy further includes receiving a current profile and
comparing the current profile with the amount of current through
the solid state switch using the current sensing network.
17. The method of claim 10, wherein releasing the stored quantity
of energy includes compensating for voltage droop on the capacitor
and variation in patient load impedance over the time interval.
18. The method of claim 10, wherein the method further comprises
determining the therapeutic quantity of energy and the time
interval to defibrillate a patient whose heart is in
fibrillation.
19. The method of claim 10, wherein the method further comprises
determining the therapeutic quantity of energy and the time
interval to pace a patient whose heart requires pacing
impulses.
20. A system to deliver a therapeutic quantity of energy to a
patient load, the system comprising: a capacitor having a rated
energy storage capacity substantially equal to the therapeutic
quantity of energy; a boost converter coupled with the capacitor
and constructed to receive a current profile and release energy
from the capacitor at a substantially constant current for a time
interval consistent with the received current profile; and an
H-bridge circuit coupled with the boost converter and constructed
to apply the substantially constant current in a biphasic voltage
waveform to the patient load.
Description
BACKGROUND
1. Field
Aspects of embodiments relate generally to methods and apparatus
for applying a selected energy impulse to a load without exceeding
a safe power level. More particularly, aspects of embodiments
relate to applying electrical energy impulses to a patient for
therapeutic medical purposes. Even more particularly, aspects of
embodiments relate to such methods and apparatus as used in heart
defibrillators and/or pacing devices.
2. Discussion of Related Art
Current defibrillator technology stores electrical energy on a
capacitor, a passive energy storage element, preparatory to
applying a timed, e.g., 10 msec, rectilinear, biphasic energy
impulse of a desired magnitude to a patient. In a known
defibrillator, energy is applied as a current from the capacitor
through the patient in a first phase, i.e., a first direction, for
the first 6 msec of the energy impulse, and then as a current also
from the capacitor, but through the patient in an opposite phase to
the first phase, i.e., in a second direction opposite to the first
direction, for the remaining 4 msec of the energy impulse. In order
to accommodate a wide range of patients and operating conditions,
especially the voltage droop that occurs as energy is transferred
out of the capacitor, the capacitor is charged to a higher level of
energy than required to produce the desired energy delivery. The
above-described, known defibrillator incorporates a resistor
network into which excess energy is dissipated by diverting a
portion of the current from being delivered to the patient when
sensors detect that power levels may be dissipated in the patient
that exceed safe power levels.
SUMMARY
According to aspects of an embodiment, a method of applying a
rectilinear biphasic electric power waveform to deliver a
therapeutic quantity of energy to treat a patient presenting an
electrical load is provided. The method comprises storing a
quantity of energy substantially equal to and without substantially
exceeding the therapeutic quantity of energy on a capacitor, and
releasing the stored energy during a first interval in a first
direction through the load presented by the patient, in a
controlled manner using a boost converter. The method may further
comprise releasing the stored energy during a second interval in a
second direction through the load presented by the patient. The
method may yet further comprise substantially exhausting the stored
energy over the first interval and the second interval combined
without exceeding a predetermined maximum safe power level when the
load presented by the patient is between approximately 25.OMEGA.
and 200.OMEGA.. The method may even yet further comprise releasing
a portion of the stored energy from the capacitor into an inductor;
releasing the portion of the stored energy from the inductor into
the load; and controlling the releasing of the portion of energy
into the inductor and into the load in an alternating sequence so
as to produce a substantially even flow of energy into the load.
According to other aspects of the embodiment, the therapeutic
quantity of energy and the first interval are selected to pace a
patient whose heart requires pacing impulses. According to yet
other aspects of the embodiment, the therapeutic quantity of energy
and the first interval are selected to defibrillate a patient whose
heart is in fibrillation.
According to aspects of another embodiment, a system for applying a
rectilinear biphasic electric power waveform to deliver a
therapeutic quantity of energy to treat a patient presenting an
electrical load is provided. The system comprises a capacitor
having a rated energy storage capacity substantially equal to the
therapeutic quantity of energy, a boost converter constructed and
arranged to meter energy out of the capacitor as a substantially
constant current while a voltage across the capacitor droops due to
decreasing energy stored on the capacitor, and an H-bridge circuit
constructed and arranged to apply the current to the patient in the
rectilinear biphasic electric power waveform. The system may
further comprise a controller that controls for a 10 msec combined
first and second interval, and a 6 msec first interval.
According to aspects of an embodiment, a system to deliver a
therapeutic quantity of energy to a patient load is provided. The
system comprises capacitor having a rated energy storage capacity
substantially equal to the therapeutic quantity of energy, a boost
converter coupled with the capacitor and constructed to release
energy from the capacitor at a substantially constant current for a
time interval, and an H-bridge circuit coupled with the boost
converter and constructed to apply the substantially constant
current in a biphasic voltage waveform to the patient load.
According to an embodiment, the boost converter comprises an
inductor coupled with the capacitor, a current sensing network, and
a solid-state switch coupled between the inductor and the current
sensing network. The boost converter may further comprise a
controller circuit coupled with the solid state switch and the
current sensing network and constructed to cycle the solid state
switch. According to other aspects of the embodiment, the current
sensing network is constructed to receive a current profile and
compare the current profile with a received current from the solid
state switch.
According to an embodiment, the H-bridge circuit comprises a
plurality of switches, each of the plurality of switches including
a circuit constructed to control the switch and to receive a phase
profile having a first phase and a second phase. According to other
aspects of the embodiment, the H-bridge circuit may further
comprise an inverter coupled with at least two of the plurality of
switches to invert the phase profile. At least two switches of the
plurality of switches may be configured to be in an open state
during the first phase and in a closed state during the second
phase.
According to an embodiment, the boost converter circuit is further
constructed to compensate for voltage droop on the capacitor and
variation in the patient load over the time interval. According to
an embodiment, the therapeutic quantity of energy and the time
interval are selected to defibrillate a patient whose heart is in
fibrillation. According to an embodiment, the therapeutic quantity
of energy and the time interval are selected to pace a patient
whose heart requires pacing impulses.
According to aspects of an embodiment, a method of delivering a
therapeutic quantity of energy to a patient load is provided. The
method comprises storing a quantity of energy substantially equal
to the therapeutic quantity of energy in a capacitor, releasing the
quantity of energy at a relatively constant current during a time
interval using a boost converter coupled with the capacitor, and
delivering a first portion of the quantity energy in a first
direction to the patient load using an H-bridge circuit coupled
with the boost converter. According to an embodiment, the method
further comprises delivering a second portion of the quantity of
energy in a second direction to the patient load using the H-bridge
circuit.
According to an embodiment, releasing the quantity of the stored
energy includes transferring energy to an inductor coupled with the
capacitor, and sensing the amount of current through a solid state
switch coupled between the inductor and a current sensing network.
According to an embodiment, releasing the quantity of energy
includes cycling the solid state switch using a controller circuit
coupled with the solid state switch and the current sensing
network. According to an embodiment, releasing the quantity of
energy further includes receiving a current profile and comparing
the current profile with the amount of current through the solid
state switch using the current sensing network.
According to an embodiment, the H-bridge circuit comprises a
plurality of switches and wherein delivering a first portion of the
quantity energy in a first direction and a second portion of the
quantity of energy in a second direction includes controlling the
plurality of switches. According to an embodiment, controlling the
plurality of switches includes receiving a phase profile having a
first phase and a second phase. According to other aspects of an
embodiment, controlling the plurality of switches further includes
changing a state of at least 4 switches of the plurality of
switches in response to receiving a change in the phase profile
from the first phase to the second phase.
According to an embodiment, releasing the quantity of energy
includes compensating for voltage droop on the capacitor and
variation in patient load impedance over the time interval.
According to an embodiment, the method further comprises
determining the therapeutic quantity of energy and the time
interval to defibrillate a patient whose heart is in fibrillation.
According to an embodiment, the method further comprises
determining the therapeutic quantity of energy and the time
interval to pace a patient whose heart requires pacing
impulses.
According to aspects of yet another embodiment, a method of
maintaining a target power flow from a charge storage device to a
patient load while voltage on the charge storage device droops,
comprises inserting a boost converter between the charge storage
device and the patient load to maintain power flow. The method may
further comprise controlling a current delivered by the boost
converter so as to compensate for voltage droop on the charge
storage device and so as to compensate for variation in patient
load impedance over time.
BRIEF DESCRIPTION OF DRAWINGS
The accompanying drawings are not intended to be drawn to scale. In
the drawings, each identical or nearly identical component that is
illustrated in various figures is represented by a like numeral.
For purposes of clarity, not every component may be labeled in
every drawing. In the drawings:
FIG. 1 is a schematic drawing of a circuit for delivering a
rectilinear biphasic electric power waveform to deliver a
therapeutic quantity of energy to treat a patient presenting an
electrical load;
FIG. 2 is a graph of electrical waveforms produced by the circuit
of FIG. 1 over a period of time with a patient load of
25.OMEGA.;
FIG. 3 is a graph of electrical waveforms produced by the circuit
of FIG. 1 over a period of time with a patient load of
50.OMEGA.;
FIG. 4 is a graph of electrical waveforms produced by the circuit
of FIG. 1 over a period of time with a patient load of
100.OMEGA.;
FIG. 5 is a graph of electrical waveforms produced by the circuit
of FIG. 1 over a period of time with a patient load of
150.OMEGA.;
FIG. 6 is a graph of electrical waveforms produced by the circuit
of FIG. 1 over a period of time with a patient load of
200.OMEGA.;
FIGS. 7A-F illustrate various current profiles and phase profiles
that may be used with the circuit of FIG. 1 to deliver a variety of
different defibrillating waveforms to the body of a patient;
FIG. 8 is a schematic drawing of a drive circuit for switches used
in an H-bridge sub-circuit of the circuit of FIG. 1;
FIG. 9 is a graph of electrical waveforms produced by the circuit
of FIG. 1 with a patient load of 25.OMEGA., when programmed for
pacer mode; and
FIG. 10 is a graph of electrical waveforms produced by the circuit
of FIG. 1 with a patient load of 300.OMEGA., when programmed for
pacer mode.
DETAILED DESCRIPTION
This invention is not limited in its application to the details of
construction and the arrangement of components set forth in the
following description or illustrated in the drawings. The invention
is capable of other embodiments and of being practiced or of being
carried out in various ways. Also, the phraseology and terminology
used herein is for the purpose of description and should not be
regarded as limiting. The use of "including," "comprising,"
"having," "containing," "involving," and variations thereof herein
is meant to encompass the items listed thereafter and equivalents
thereof as well as additional items.
As noted in the BACKGROUND section, defibrillators are devices that
deliver a desired quantity of energy to a patient without exceeding
a safe power level. Energy is simply power delivered to a load over
a period of time:
##EQU00001## ##EQU00001.2## ##EQU00001.3## where P represents power
in Watts, E represents energy in Joules, and t represents the
period of time in seconds over which the energy is delivered. When
electrical energy is dissipated in a simple resistive load, that
is, one which resists a flow of electrical current when a voltage
is applied, power may be expressed in terms of the voltage applied
to the load, voltage being a measure of electrical pressure across
the load, and current through the load, current being a measure of
movement of charge through the load. Electrical power is:
##EQU00002## ##EQU00002.2## .times. ##EQU00002.3## ##EQU00002.4##
##EQU00002.5## where V represents voltage in Volts, I represents
current in Amperes, and R represents the resistance of the load in
Ohms.
Defibrillators store the desired quantity of electrical energy on a
capacitor, as a charge. Storing a charge on a capacitor causes a
voltage to appear across the terminals of the capacitor. When a
user of a defibrillator applies a therapeutic shock to a patient,
the electrical energy stored on the capacitor is released through
the patient, whose body provides substantially a simple resistive
load in which the energy is dissipated. As the capacitor supplies
energy to the load, the charge on the capacitor decreases, and so
the voltage appearing across the capacitor also decreases. As
voltage decreases, or sags, the current driven through the load
also decreases. Applying any of the definitions of electrical power
given above, it is observed that the power, P.sub.0, delivered by
the capacitor at the beginning of a therapeutic shock of a defined
magnitude, E.sub.0, is greater than the power, P.sub.N, delivered
by the capacitor at the end of the therapeutic shock because the
voltage on the capacitor sags as the charge on the capacitor is
depleted by supplying current to the patient.
Conventionally, in order to accommodate the voltage sag, while
delivering a constant, desired maximum power level until the
desired energy impulse has been delivered, the size of the
capacitor is selected to provide the desired energy impulse to a
worst-case load at the end of the energy impulse. For these
purposes, a worst-case load may be considered to be one at a lower
end of an expected resistance range, since such a load will require
a larger current to maintain a constant power level during the
energy impulse. Such a design requires a capacitor that, when
charged to a level that yields the desired energy impulse,
dissipates in the patient a power level in excess of that desired
during the initial portion of the energy impulse. As previously
explained, during times of excess power delivery, the excess energy
is simply dissipated into resistors so as to reduce to desired
maximum levels the power delivered to the load, i.e., the patient,
which both wastes power and necessitates the use of a capacitor
whose rated energy storage capacity is greater than the maximum
energy delivery requirement, since energy is dumped into the
dissipation resistors and not recovered or otherwise put to
therapeutic use.
Using a capacitor whose rated energy storage capacity is greater
than the maximum energy delivery requirement is disadvantageous
from several perspectives. For a given capacitor technology,
greater storage capacity requires greater size and/or weight. A
physically larger capacitor is undesirable, particularly for use in
portable equipment, because equipment must be built larger and is
more difficult to transport. Size and weight factors can prove
prohibitive for equipment meant to be worn by, transported with, or
carried by, a patient who themselves may not be fully ambulatory.
Moreover, energy that is wasted, yet must be stored on the
capacitor as described above, adds to the charging time and the
performance characteristics required of the charging circuit which
places the energy on the capacitor.
For example, in the conventional defibrillator described in the
BACKGROUND section in which excess energy is dissipated into
dissipation resistors, a capacitor having a minimum required energy
rating of approximately 381 Joules is used. Under favorable
conditions for maximum energy shock (i.e., a 200 Joule setting into
a patient presenting an impedance of 161.OMEGA.), approximately 69%
of the capacitor's minimum required energy rating is delivered to
the patient. For higher impedance patients, energy utilization
drops off slightly to 67% for a patient presenting an impedance of
175.OMEGA., and to 63% for a patient presenting an impedance of
200.OMEGA.. The drop off in energy utilization is more severe for
lower impedance patients (e.g., 37% for a patient presenting a
25.OMEGA. impedance, and 21% for a patient presenting a 15.OMEGA.
impedance), primarily due to energy dissipated in the dissipation
resistors. One of the physically smallest capacitors validated for
use in such a conventional defibrillator weighs approximately 10 oz
(283.5 grams) and has a volume of approximately 20 in.sup.3 (327.7
cm.sup.3).
According to aspects of embodiments, a boost converter is employed
to control and regulate the delivery of a constant current,
resulting in a constant power dissipation level during the delivery
of a desired energy impulse. In brief summary, a boost converter
transfers energy in very short bursts compared to the time for
delivering the total desired energy impulse, first from the
capacitor to an inductor, which stores the energy as a
substantially constant current, and then from the inductor to the
patient. Because the current delivered to the patient by the
inductor is substantially constant due to the intrinsic electrical
characteristics of inductors which tend to resist a change to
current through them, a constant, maximum desired power level is
dissipated in the patient, in accordance with the definitions of
electrical power given above. A boost converter circuit of a
defibrillator incorporating aspects of embodiments is now described
in greater detail.
First, the basic boost converter circuit is described in connection
with FIG. 1. The boost converter circuit, 100, provides a
substantially constant current at its output node, 101, when that
node is connected to a load, 102. The circuit, 100, includes a
storage capacitor, 103, in which the energy for the desired impulse
is held until a discharge into the patient load is triggered; an
inductor, 104, connected to receive a current from the storage
capacitor, 103, when the discharge is triggered; a diode, 105, to
protect against a reversal of the current discharge; and,
optionally a smoothing capacitor, 106; as well as control elements
enumerated below. A charge circuit, such as a battery or other DC
power source (not shown) is coupled to the storage capacitor 103,
for example via relays, to provide energy to the storage capacitor
103. A terminal of the storage capacitor 103 is electrically
coupled to a first terminal of the inductor 104, which in FIG. 1 is
modeled as an inductor 1041 coupled in series with a resistor 104r.
The second terminal of the inductor 104 is electrically coupled to
the anode of the diode 105, with the cathode of the diode 105 being
electrically coupled to a first terminal of the optional smoothing
capacitor 106 and to the output node 101.
According to the capacitor energy equation, E=1/2CV.sup.2, an
exemplary capacitor, 103, of 270 .mu.F, as shown in FIG. 1, charged
to about 1218 V would store 200 Joules for the exemplary
therapeutic shock. To deliver 200 Joules over a 10 msec impulse
requires delivering a substantially constant, instantaneous power
of 20 kW to the load, 102. If the load, 102, is 25.OMEGA., then the
power equation, P=I.sup.2R, calls for a current of 28 A, while a
load, 102, of 200.OMEGA. calls for a current of 10 A. Inductor,
104, is of a size to prevent substantial current droop while
delivering a desired power level to the patient load, 102. A 1 mH
inductor, 104, as shown in FIG. 1, produces the desired result, as
illustrated below in FIGS. 2-6. In accordance with one embodiment,
the inductor 104 may be an unsaturable 1 mH Litz wire air-core coil
dimensioned to optimize self inductance.
While 100% utilization of the capacitor energy storage capability
is the theoretical goal, practical circuit elements, which have
real losses associated with them, achieve somewhat lower
utilization rates, per the Table I, below. The simulations
presented in FIGS. 2-6, and discussed below, assume capacitor, 103,
has a capacitance of 270 .mu.F, and an initial stored energy of 305
Joules.
TABLE-US-00001 TABLE I Patient Therapeutic Shock 10 msec Continuous
Initial Energy Impedance Energy Power Usage 25 .OMEGA. 249 Joules
24.9 kW 81% 50 .OMEGA. 255 Joules 25.5 kW 83% 100 .OMEGA. 236
Joules 23.6 kW 77% 150 .OMEGA. 221 Joules 22.1 kW 72% 200 .OMEGA.
210 Joules 21.0 kW 69%
By comparison to a conventional defibrillator using a storage
capacitor having a minimum energy rating of 381 Joules, embodiments
of the present invention permit the use of a storage capacitor
having an approximately 20% lower minimum energy rating (e.g., 305
Joules) while providing a similar amount of energy to the patient.
As a result, the size and weight of the storage capacitor 103 used
with embodiments of the present invention may be reduced by
approximately 20% relative to storage capacitors used in a
conventional defibrillator. Further efficiencies of size and cost
are provided by eliminating the need for dissipation resistors and
their associated shunting devices used in conventional
defibrillators, as well as any of the thermal management features
needed to dissipate the heat generated therefrom.
A specialized controller circuit, 107, modeled for convenience as a
UC3842 current mode PWM controller, has a control output connected
to a control input of a high-voltage and high-current, solid-state
switch, 108 that is coupled between the second terminal of the
inductor 104 and a current sensing network 109. The solid state
switch 108 may be an IGBT as shown in FIG. 1, or another type of a
high-voltage and high current solid state switch, such as a
thyristor. It should be appreciated that embodiments of the present
invention are not limited to the use of a particular type of PWM
controller or to a particular type of high-current solid state
switch, as other types of controller circuits, and other types of
high-current switches may alternatively be used. Current drawn
through the switch 108 is measured by the current sensing network,
109; compared to a desired current profile, 110; and, the result is
provided as an input to the controller circuit, 107. Since, as
explained above, current is directly related by a square law to
instantaneous power, controlling for a desired current also
controls for the desired instantaneous power level.
The load presented by the patient, 102, is connected to the output
node, 101, through an H-bridge structure which causes current to
flow through the patient in a desired direction at a desired time.
The H-bridge includes four H-bridge switches 111, with each
H-bridge switch 111a, 111b, 111c, 111d including a respective
switching transistor 116a, 116b, 116c, 116d and a respective
control circuit 117a, 117b, 117c, 117d associated with each. The
switching transistors can be insulated-gate bipolar transistors
(IGBTs), metal-oxide semiconductor field-effect transistors
(MOSFETs), silicon-controlled rectifiers (SCRs) or such other
high-current switching devices as may be available. In the
exemplary, illustrative embodiment, for modeling purposes only, an
oscilloscope, 112, having a channel A input, 112A, and a channel B
input, 112B, has been included. Channel A, 112A, monitors the
current impulses passed through the switch, 108, and channel B,
112B, monitors the voltage across the patient load, 102. The traces
produced by channels A and B, 112A and 112B, are shown in FIG. 2,
which is next referred to in an explanation of the operation of the
circuit of FIG. 1.
The circuit of FIG. 1 operates as follows to provide a 200-Joule
defibrillation shock to a patient load, 102, of 25.OMEGA., as
illustrated in FIG. 2. The storage capacitor, 103, is first charged
up with about 200 Joules of electrical energy, by a charging
circuit (not shown). There are small, parasitic losses due to
parasitic resistances throughout the circuits which deliver the
charge to the patient, including parasitic resistances in the
inductor, 104r, and elsewhere. If the parasitic resistances are
negligible, then no more than about 200 Joules need be stored on
the capacitor; however, if the parasitic resistances are
non-negligible, then the storage capacitor, 103, should hold a
small excess above the desired 200 Joules of electrical energy, the
excess being sufficient to just account for the energy dissipated
in the parasitic resistances under worst-case conditions. It should
be appreciated that other shock energies (i.e., other than 200
Joules) may be provided, as known to those skilled in the art.
According to one embodiment, operation begins with the solid-state
switch, 108, open, and each of the H-bridge switches, 111a-d, open.
When a therapeutic shock is triggered, a pair of the H-bridge
switches, e.g., 111a and 111c, is closed, initiating current
through the patient load 102. Current then builds up in the
inductor, 104. As shown in FIG. 2, indicated by line, 200, current
through switch, 108, is zero during this initial period, 201. Next,
during period, 202, the controller circuit, 107, begins cycling
switch, 108, on and off, thereby allowing current through switch,
108. When the switch, 108, is closed and the controller circuit,
107, detects that the desired current or higher is flowing through
the switch, 108, it provides a control signal to the solid-state
switch, 108, to again open the switch, 108, allowing current
through the inductor, 104, to the patient load, 102. At regular
intervals, the controller, 107, closes the switch, 108, and checks
for the current to build up to the desired level, at which point
the control signal again opens the switch, 108.
During each cycle, during period, 202, when the controller circuit,
107, determines from the output of the current sensing network,
109, that the correct current level has been reached or exceeded, a
control signal is applied to the solid-state switch, 108, to open
the switch, allowing current through the inductor, 104, and the
patient load, 102, from the energy stored on storage capacitor,
103. As current is initiated through the patient, a voltage,
indicated in FIG. 2 by line 210, appears across the patient that
causes the current in the inductor, 104, to begin to decay, and so
the controller circuit, 107, again closes solid-state switch, 108,
to begin the cycle again by building up the current stored in the
inductor. By repeating the forgoing cycle many times during the
therapeutic shock, energy stored on the storage capacitor, 103, is
metered out to the patient without ever exceeding the maximum
allowable power dissipation level in the patient. According to some
embodiments, it has been found that the desired waveform to be
applied to the patient reverses polarity after an interval.
Accordingly, the H-bridge switches are controlled by a desired
phase profile, 120, to open the closed pair of switches, 111a and
111c, and close the open pair of switches, 111b and 111d, at about
6 msec into the therapeutic shock cycle, reversing the polarity of
the applied shock, 211. It should be noted that switches 111a and
111c are opened prior to closing switches 111b and 111d to avoid
short-circuiting the H-bridge structure. The magnitude of the
current applied to the patient load, 102, (and the resulting
voltage across the patient load, 102) does not substantially change
during the polarity reversal, 211 and 213.
As shown in FIGS. 3, 4, 5, and 6, the operation is similar for
patients presenting resistance values of 50.OMEGA., 100.OMEGA.,
150.OMEGA., and 200.OMEGA.. The variation in the load, 102, results
in different damping characteristics for the therapeutic shock
waveforms, i.e., the overall shape of the waveform, and also
results in different patient voltages, such that the 200 Joule
impulse is applied as desired. In each of FIGS. 3, 4, 5, and 6,
reference numerals indicating corresponding elements to elements of
FIG. 2 correspond, except for the hundreds place, which corresponds
to the FIG. number. For example, FIG. 2, line 200, corresponds to
FIG. 3, line 300, but for a different patient load, 102.
As shown in FIG. 2, for a patient load, 102, of 25.OMEGA., the
absolute value of patient voltage 210 varies between a peak of
about 1.5 kV and 500 V. For a patient load, 102, of 50.OMEGA., the
absolute value of patient voltage 310 has a much flatter shape, as
shown in FIG. 3. It hits a peak of about 2 kV but remains for most
of the impulse at about 1.2 kV, finally tapering down to just under
1.0 kV. As shown in FIG. 4, for a patient load, 102, of 100.OMEGA.,
the absolute value of patient voltage 410 has an even flatter
shape. It also hits a peak of about 2 kV but remains for most of
the impulse at about 1.5 kV, finally tapering down to about 1.2 kV.
For a patient load, 102, of 150.OMEGA., the absolute value of
patient voltage 510 has a quite flat shape, as shown in FIG. 5. It
hits a peak of about 2 kV but remains for nearly the entire impulse
at about 1.8 kV. For a patient load, 102, of 200.OMEGA., the
absolute value of patient voltage 610 has a quite flat shape. It
hits a peak of about 2 kV but remains for nearly the entire impulse
at about 1.8 kV, as shown in FIG. 6.
In practical systems, the preference is to deliver substantially
constant energy to the patient during a period of time. Thus, if a
200 Joule therapeutic shock is desired to be delivered in a 10 msec
period, the controller circuit, 107, is designed or programmed to
obtain a current level in the inductor, 104, that delivers 20
J/msec. The controller circuit, 107, in connection with the current
sensing network, 109, the desired current profile, 110, and the
solid state switch, 108, forms a feedback loop that controls and
maintains the 20 J/msec level, or such other level or waveform as
desired. Except for parasitic losses, explained below, the storage
capacitor, 103, need only have a rated energy storage capacity of
200 Joules, since no excess energy is dumped and it is desired to
leave no residual energy in the storage capacitor, 103, after the
therapeutic shock has completed.
FIGS. 7A-F illustrate the manner in which the boost converter
circuit 100 of FIG. 1 may be used to control the shape and/or phase
of the defibrillating waveform applied to the patient. As described
previously with respect to FIG. 1, current drawn through the switch
108 is measured by the current sensing network 109, compared to a
desired current profile 110, and provided as an input to the
controller circuit 107. In the circuit illustrated in FIG. 1, this
comparison is performed by an operational amplifier 118 configured
as a comparator. By controlling the shape and amplitude of the
desired current profile 110, a waveform of a desired amplitude and
desired shape may be delivered to the patient load 102.
For example, as shown in FIG. 7A, a current profile 110 having a
step impulse that corresponds to a current level of approximately
22 A and that gradually increases to a level corresponding to
approximately 40 A may be used to provide the current and voltage
waveforms depicted in FIGS. 3-6. The current profile 110 is
approximately 10 msec in duration and includes an initial period or
region 110a where the current is zero (corresponding to periods
301, 401, 501, and 601 in FIGS. 3-6), followed by a step impulse
(region 110b) corresponding to approximately 22 A. At approximately
1 msec, the current profile 110 linearly increases (region 110c) to
a value corresponding to approximately 24 A. The current profile
110 remains at a level corresponding to about 24 A for about 2 msec
(region 110d), where it then linearly increases (region 110e) to a
level corresponding to approximately 40 A, after which the current
profile returns to a zero current level (region 110f) at
approximately 10 msec. The overall shape of the current profile 110
used to generate the current and voltage waveforms depicted in
FIGS. 3-6 is shown in each respective figure by the respective
envelope or profile 303, 403, 503, and 603 of the current through
the switch 108. Differences in the voltage waveforms 310, 410, 510,
and 610 applied to the patient in FIGS. 3-6 are primarily due to
differences in the patient load 102. The overall shape of the
current profile 110 used to generate the current and voltage
waveforms depicted in FIG. 2 is shown in FIG. 2 by the envelope 203
of the current 200 through the switch 108.
The phase of the defibrillating waveform that is applied to the
patient may be controlled by the desired phase profile 120 provided
to each of the H-bridge switches 111. For example, FIG. 7B
illustrates a desired phase profile 120 that may be used to control
the phase of the waveforms depicted in FIGS. 2-6. As shown, the
phase profile 120 initially assumes a high state at time zero (or
before) followed by a change to a low state at 6 msec. In the
circuit depicted in FIG. 1, each of H-bridge switches 111a and 111c
receives the phase profile 120, while each of H-bridge switches
111b and 111d receives an inverted version of the phase profile
120. The high state of the phase profile 120 operates to fully
close each of H-bridge switches 111a and 111c during the initial 6
msec, and to maintain each of switches 111b and 111d in a fully
open position. At approximately 6 msec, the level of the phase
profile 120 changes, thereby fully opening H-bridge switches 111a
and 111c, and fully closing switches 111b and 111d, thereby
reversing the polarity of the delivered voltage waveform as shown
in each of FIGS. 2-6. The presence of the inverter 115 serves to
not only invert the phase profile 120, but to delay the signal
provided to each of H-bridge switches 111b and 111d to help ensure
that these switches are not closed until after switches 111a and
111c have opened. If necessary, additional delays could be
provided. Where the switching transistors 116a, 116b, 116c, and
116d used in each of the H-bridge switches 111a, 111b, 111c, 111d
are capable of operating in a linear mode, the high state and the
low state of the phase profile 120 should be such that the
switching transistors are either fully conducting (on) or fully
non-conducting (off) to avoid thermal destruction.
FIG. 7C illustrates an alternative desired current profile 110 that
may be used with the boost converter circuit 100 of FIG. 1 to
generate a biphasic voltage waveform that increases asymptotically
from a zero value to a desired voltage level (e.g., to
approximately 2 kV in amplitude for a 200.OMEGA. patient) over the
initial portion of each phase. When combined with a phase profile
110 similar to that illustrated in FIG. 7B, the boost converter
circuit 100 may provide a defibrillating voltage 710 to the body of
the patient similar to that shown in FIG. 7D. As shown in FIG. 7D,
the biphasic voltage waveform 710 is approximately 10 msec in
duration and switches phase at approximately 6 msec. During the
first few milliseconds of each phase, the voltage 710 applied to
the body of the patient rises asymptotically to an amplitude of
about 2000 V (for a 200.OMEGA. patient). To achieve the shape of
the voltage waveform 710 shown in FIG. 7D, the smoothing capacitor
106 (FIG. 1) may be omitted or set to a value of zero. Such a
ramped asymptotic voltage waveform as shown in FIG. 7D may reduce
the amount of trauma to the patient's heart during defibrillation,
by avoiding the step impulse in voltage shown in each of FIG.
2-6.
FIGS. 7E and 7F illustrate alternative phase profiles that may be
used with the current profile 110 discussed above with respect to
FIG. 7A. For example, FIG. 7E illustrates a desired phase profile
120 having two opposing phases over a 10 msec duration, in which
the opposing phases are substantially similar in duration (i.e.,
about 5 msec each). Such a phase profile may be used to balance the
amount of charge delivered to the patient's heart in each direction
and thereby potentially reduce the trauma to the patient's heart.
FIG. 7F illustrates yet an alternative phase profile that may be
used to provide a monophasic defibrillating shock to the body of
the patient.
It should be appreciated that a variety of different current
profiles 110 and phase profiles 120 may be used with the boost
converter circuit 100 of FIG. 1 to generate a corresponding variety
of different defibrillating waveforms, each having a different
shape and/or amplitude and/or phase. For example, the current
profile 110 could include a simple exponential waveform or a damped
sine wave, and the phase profile 120 could be monophasic, biphasic,
triphasic, or otherwise. Accordingly, where it is determined that a
particular shape, amplitude, phase, sequence of phases, or all of
the above is particularly effective, reference waveforms may be
generated and used as desired current profile 110 and desired phase
profile 120 to achieve the desired resultant defibrillating
waveform. Although not depicted in FIG. 1, each of the desired
current profile 110 and the desired phase profile 120 may be stored
in a memory, and provided, for example, by a processor of the
defibrillator to comparator 118 and each of H-bridge switches 111
to vary the shape, amplitude, or phase of the defibrillating
waveform applied to the patient as desired.
Referring back to FIG. 1, again, within each H-bridge switch,
111a-d, is a driver circuit (i.e., 117a, 117b, 117c, and 117d).
These circuits are feedback systems illustrated in greater detail
in FIG. 8. Each driver circuit is configured to control a switching
transistor (i.e., 116a, 116b, 116c, and 116d, respectively) to pass
a current up to a controlled maximum level above which the current
is clipped.
In each driver circuit is an operational amplifier, 802, connected
to receive a control signal, DRIVE, that is based upon the desired
phase profile 120 and produce an output, GATE, which turns a
switching transistor (FIG. 1, 116a, 116b, 116c, and 116d) on and
off as required. A second operational amplifier, 804, is connected
to sense current through each switching transistor (FIG. 1, 116a,
116b, 116c, and 116d), and control an input to operational
amplifier, 802, so a voltage at output, GATE, turns the switching
transistor on up to a desired maximum current through the switching
transistor. In the embodiment depicted in FIG. 1, each of the
switching transistors 116a, 116b, 116c, and 116d is an IGBT that
may be operated in a non-linear mode as a two state (i.e., on or
off) switch, or in a linear mode as a voltage controlled current
source. Where the circuit 100 is used to provide therapeutically
effective amounts of energy sufficient for defibrillation, the
switching transistors 116a, 116b, 116c, and 116d would typically be
operated in the non-linear mode (e.g., as a two state switch) to
avoid thermal destruction. However, by adjusting the timing defined
by controller circuit, 107, the desired current profile, 110, and
the desired phase profile, 120, the circuit of FIG. 1 can be
programmed to perform pacing, as well as defibrillation. For
example, by appropriately controlling the timing defined by the
controller circuit 107 and the desired current profile 110, and at
energies typically used for pacing, the smoothing capacitor 106 can
substantially correspond to a DC power supply of a desired voltage.
By then providing a suitable phase profile to pairs of H-bridge
switches (i.e., 111a and 111c, 111b and 111d), the switching
transistors of a respective pair of H-bridge switches may be
operated in their linear mode to provide a desired pacing
waveform.
FIGS. 9 and 10 illustrate a phase profile 120 that may be provided
to pairs of H-bridge switches, 111a and 111c or 111b and 111d, to
deliver suitable pacing waveforms 902, 1002, to a patient load 102,
when the timing defined by the controller circuit 107 and the
current profile 110 have been appropriately programmed In each of
FIGS. 9 and 10, the timing of the controller circuit 107 and the
current profile 110 have been programmed so that smoothing
capacitor 106 effectively operates as a 50 V D.C. voltage source.
As illustrated in FIG. 9, a voltage level of approximately 27 mV
provided to the DRIVE input of driver circuits 117A and 117C is
sufficient to deliver an 8 mA, 200 mV pacing pulse into a patient
102 presenting a 25.OMEGA. load. As illustrated in FIG. 10, a
voltage level of approximately 465 mV is sufficient to deliver an
140 mA, 42 V pacing pulse into a patient 102 presenting a
300.OMEGA. load. By appropriately controlling the phase profile
120, pacing pulses ranging from a few milliamps to two hundred
milliamps or more may be provided to a patient, using the same
circuit topology as that used to deliver a defibrillating
shock.
It should be appreciated that although embodiments of the present
invention have been primarily described with respect to
defibrillation and pacing, they may also be used to deliver other
types of therapeutic waveforms to the body of a patient in which
the energy delivered is between the ranges of energy typically used
for pacing or defibrillation. For example, pacing pulses typically
range from a few mA to approximately 200 mA, and defibrillation
pulses typically range from about 1 A to about 35-40 A. Between
these ranges of current exists a wide spectrum of energies that may
be applied to the body of a patient for a variety of therapeutic
purposes, for example, to perform charge bumping of a patient's
heart, etc. Accordingly, by varying the timing of the controller
circuit, the current profile 110, and the phase profile 120,
embodiments of the present invention may be used to tailor one or
more of the shape, the voltage, and the current of a therapeutic
waveform to be applied to the body of a patient.
Having thus described several aspects of at least one embodiment of
this invention, it is to be appreciated various alterations,
modifications, and improvements will readily occur to those skilled
in the art. Such alterations, modifications, and improvements are
intended to be part of this disclosure, and are intended to be
within the scope of the invention. Accordingly, the foregoing
description and drawings are by way of example only.
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